The Internet Protocol Interoperability and Communications System (IPICS) available from Cisco Systems, Inc., San Jose, Calif., USA, which enables communications between multiple devices, leverages multicast networks as a means to support the media routing between endpoints participating in a virtual talk group (VTG). In cases where the multicast routing (i.e., simultaneously sending information packets to a group of destinations) is not feasible, the endpoints send and receive media over unicast (i.e., sending information packets to a single destination). A typical VTG carries a mix of multicast and unicast traffic.
The IPICS interfaces with Land Mobile Radios (LMRs) via a Land Mobile Radio Gateway (LMRG). This gateway simply maps one multicast group in the IP network to an Ear/Earth (E) & Mouth/Magnet (M) interface that is then connected to the radio. The IPICS uses this mechanism to receive and transmit audio over the radio network. Even though a radio can be tuned to a multitude of channels, all channels share a single multicast address. This is a significant limitation in terms of access control and interoperability to the IPICS. For instance, assume a radio could be tuned to the fire department frequency as well as to the police frequency; the IPICS administrator would then like to provision some users with access only to the police channel and not to the fire department channel. This is not possible on the current system because a single multicast address is shared by the two channels on the same radio, and most of the IPICS's endpoints are only able to differentiate channels on the basis of different multicast addresses.